Processes of the above-indicated type are well known in the art and such processes are described e.g. in U.S. patent specifications No. 4,279,873 and 4,197,278 and in the references cited in said specifications all of which are hereby incorporated by reference.
The absorbent which finds widest application in these desulfurization processes is slaked lime, i.e. calcium hydroxide possibly containing some magnesium hydroxide, but also sodium carbonate and other basic substances come into consideration.
These processes result in a by-product which is a powdery material the composition and structure of which depend on several features such as amount and composition of fly ash in the flue gas to be desulfurized and composition of the selected absorbent, and on operational features such as the temperature in the absorption zones. The use of partial recycling of the by-product for use in the preparation of the absorbent liquid to be atomized, highly influence the structure of the by-product purged from the process.
The sulfur which is removed from the flue gas will be present in the powdery by-product as sulfite and sulfate. Usually the proportion of sulfite is somewhat higher than the proportion of sulfate.
This relatively high sulfite content whether present as sodium, magnesium or calcium sulfite, may however cause some restrictions as to the disposal or industrial utilization of the by-product. Sodium sulfite being highly soluble in water represents a potential risk to subsoil water when disposed and, although calcium sulfite is only slightly water soluble its disposal in nature has somewhere been met with the reluctance of the authorities. Also for the utilization in the cement industry of by-products containing fly ash and obtained using lime as absorbent, the high calcium sulfite content represents a restriction.